Characterization of the molecular basis for virus attachment to cells has importance both for understanding virus tropism and for developing agents that inhibit virus binding or alter the specificity of binding. Recently, a cellular receptor for adenovirus type 2 and other closely related serotypes was identified. This receptor, encoded by a single gene on human chromosome 21 (Mayr et al., J. Virol. 71: 412–8 (1997)), is a 46 kD glycoprotein which also serves as a receptor for group B coxsackieviruses (CBV) and thus was termed the coxsackievirus and adenovirus receptor (CAR). CAR mRNA is present in many human tissues. A broad tissue distribution of CAR protein expression correlates with the broad tropism of CBV, but subgroup C adenoviruses that are known to bind CAR have a much more restricted tropism limited primarily to the upper respiratory tract. Thus, other factors in addition to receptor availability clearly have important roles in determining adenovirus tropism. Although adenovirus binds to CAR with high affinity (Mayr et al., J. Virol. 71: 412–8 (1997); Wickham et al., Cell. 73: 309–19 (1993)), virus titers are significantly reduced on cells with down-regulated CAR expression (Freimuth, P., J. Virol. 70: 4081–5 (1996)). These results suggest that adenovirus infection in vivo may be restricted to cells which express CAR at levels above a minimum threshold concentration. CAR protein levels are relatively low on the apical surface of differentiated (ciliated) respiratory epithelial cell cultures, which may account for the poor efficiency of adenoviral gene transfer to human lung tissue in vivo.
Adenovirus binding to CAR results from an interaction between rod-shaped proteins located at the capsid vertices, called viral fibers, and the extracellular region of CAR. The monomers of this homotrimeric fiber protein range in size from 30 to 65 kDa depending on the serotype (Huang et al., J. Virol. 73: 2798–2802 (1999)). They are composed of a conserved amino terminal tail that mediates their interaction with the penton base, a variable-length elongated (shaft) domain, and a carboxyl-terminal globular domain, termed the knob, which mediates the high-affinity interaction with its cellular receptor. The knob domain of adenovirus type 5 (Ad5) was expressed in E. coli as a soluble, trimeric, biologically active protein, and its 3-dimensional structure was determined by x-ray crystallography (Xia et al., Structure 2: 1259–70 (1994)).
The predicted amino acid sequence of CAR suggests a structure consisting of two extracellular domains related to the immunoglobulin IgV and IgC2 domain folds (Bork et al., J. Mol Biol. 242: 309–20 (1994); Bergelson et al., Science 275: 1320–3 (1997); Tomko et al., Proc. Natl. Acad. Sci. USA 94: 3352–6 (1997)), a single membrane-spanning region, and one carboxy-terminal cytoplasmic domain. Regions of CAR necessary for binding the fiber knob domain have not yet been determined.